1. Field of the Invention
The present invention relates to a casting apparatus having two or more coating layers, which is suitable for the casting of low-melting metals including aluminum, zinc, tin, and alloys thereof.
2. Description of the Prior Art
The casting of low-melting metals requires such apparatus as a stalk, ladle, protecting tube for the thermocouple, molten metal stirring blade, and a gas blowing tube. It is conventional practice to make these items from metal such as ferrous alloy, refractory, or ceramics.
Those apparatus which are used for the casting of aluminum alloys are usually made of, for example, cast iron, which offers high strength, low price, and good formability. Cast iron, however, has the disadvantage of being susceptible to corrosion from molten aluminum. Upon corrosion, the cast iron dissolves into the molten aluminum, which leads to contamination and deterioration of the cast aluminum.
In order to solve this problem, the use of ceramic apparatus or the coating of cast iron apparatus has been proposed.
For example, Japanese Patent Laid-open Application (KOKAI-KOHO) No. 180657/1985 discloses a ceramic stalk for low-pressure casting which is made from metallic silicon, alone or together with refractory aggregates, by nitriding sintering. The ceramic stalk prevents contamination. This merit, however, is offset by the fact that it is ten times more expensive than a cast iron stalk and that it is easily broken under load and hence has a rather short life for its high price. In addition, it is difficult to make casting apparatus of complex shapes from ceramics.
Also, Japanese Patent Laid-open No. 6772/1981 discloses the formation of a corrosion-resistant boron nitride coating on casting apparatus in contact with molten aluminum or aluminum alloy. Although boron nitride itself provides good corrosion resistance, the boron nitride coating does not provide sufficient durability because of its poor adhesion to the substrate.
A common practice to impart durability is the use of a binder. A binder for a coating on a ferrous substrate should meet the following requirements.
It does not rust the substrate during fabrication.
It should have a coefficient of thermal expansion close to that of a ferrous base metal.
It should have good adhesion to the substrate.
It should have good resistance to oxidation.
Common materials meeting these requirements are silicates and a variety of glass powders, including borosilicate glass, having a melting point below 1000.degree. C.
There was proposed in Japanese Patent Application No. 300250/1989 [Japanese Patent Laid-open Application (KOKAI-KOHO) No. 161162/1991] a corrosion-resistant coating material containing a fluorine compound having a melting point higher than 700.degree. C. According to the disclosure, this coating material is used to form a single coating layer on casting apparatus for low-melting metals. However, the use of a binder is restricted by the substrate to which it is applied. Moreover, it is inconceivable that corrosion resistance varies depending on the kind of a binder.
In other words, a fluorine compound present in a single coating layer containing either an alkaline component such as silicate or a low-melting component such as borosilicate glass will undergo reduction and degrade the coating material. Therefore, the binder will not contribute to the long-term durability of the coating layer.
It is known that a coating layer does not react with low-melting metals if it contains a fluorine compound and employs a highly heat resistant binder in the form of sol such as silica sol and alumina sol. Such a coating layer, however, offers little long-term durability because its adhesion to the substrate resorts to mechanical bonding and hence, it is subject to peeling from thermal and mechanical shocks.